Revised Manuscript 15 Ianuarie (1) [611588]

1 CoFe 1.8RE 0.2O4 (RE3+ = Tb3+, Er3+) Ferrite Nanoparticles: Synthesis, Characterization
and Biological Properties

Ioana Mindru1, Dana Gingasu1,*, Adelina -Carmen Ianculescu1, Vasile Adrian Surdu2,
Gabriela Marinescu1, Silviu Preda1, Cristina Bartha3, Marcela Popa4, Mariana Carmen
Chifiriuc4

1“Ilie Murgulescu ” Institute of Physical Chemistry, Romanian Academy, Splaiul Independentei
202, Bucharest 060021, Romania
2“Politehnica” University of Bucharest, Faculty of Chemistry, Polizu Street, no. 1 -7, Bucharest,
Romania
3National Institute of Materials Physics, Atomistilor Street, No. 405 A, P. O. Box Mg -7, Bucharest –
Magurele 077125, Romania
4University of Bucharest, Faculty of Biology, Microbiology Department; Research Institute of the
University of Bucharest, ICUB, Life, Environmental and Earth Sciences Division, Splaiul
Independentei 91 -95, Bucharest, Romania

*Corresponding author.
E-mail address : [anonimizat] (D. Gingasu)

2 Abstract Nanosized CoFe 1.8RE 0.2O4 (RE3+ = Tb3+, Er3+) ferrites were obtained through wet
ferritization method . The se ferrites were characterized by X-ray diffraction (XRD),
scanning electron microscopy (FE -SEM), transmission electron microscopy (TEM/HR –
TEM), Fourier transform infrared spectroscopy (FTIR) and magnetic measurements. The
XRD results revealed that the av erage crystallite size is 5.77 nm for CoFe 1.8Tb0.2O4 and
6.42 nm for CoFe 1.8Er0.2O4. Distribution of metal cations in the spinel structure estimated
from X -ray diffraction data showed that the Tb3+ and Er3+ ions occupy the octahedral sites .
TEM images indicated the presence of polyhedral p articles with average size 5.91 nm for
CoFe 1.8Tb0.2O4 and 6.80 nm for CoFe 1.8Er0.2O4. The saturation magnetization value (M s) is
60 emu/g for CoFe 1.8Tb0.2O4 and 80 emu/g for CoFe 1.8Er0.2O4. CoFe 1.8RE 0.2O4
nanoparticles showed simil ar antimicrobial efficacy against the five tested microbial
strains, both in planktonic and biofilm state. The results highlight the promising potential of
these types of nanoparticles for the development of novel anti -biofilm agents and materials.

Keywords : Wet chemical method ; Ferrite; Spinel structure ; Antimicrobial activity .

1 Introduction

Spinel ferrite and substituted spinel ferrite nanoparticles have attracted an increasing
interest, in the last decades, due to their potential applications in several fields, such as
perma nent magnets, microwave devices, information st orage systems and magnetic fluids
[1]. In terms of biomedical applications , they are used for diagnosis ( magnetic resonance
imaging, biosensors ) and treatment (e.g., cancer treatment , drug delivery systems) [2-7].

3 The properties of these ferrites are dependent on the nature of the cations, their
charges and their distribution between the tetrahedral ( A) and the octahedral ( B) sublat tices
of the spinel structure [8].
Cobalt ferrite (CoFe 2O4), an important membe r of the spinel family, crystallizes in a
partially inverse spinel structure 
where  is the degree of
inversion and it depends on the preparation c onditions and thermal history [9]. CoFe 2O4 is
ferrimagnetic with a Cu rie temperature (T c) around 520 C, large anisotropy and moderate
saturation magnetization, as well as tunable coercivity; it exhibits physical and chemical
stability [10].
Seve ral reports have been published on the effect of the substitution of Co2+ in
cobalt ferrite with rare earth and transition metal ions .
The presence of rare earth elements influe nces its structural and magnetic
properties. The large ionic radii and highly localized magnetic moments of 4f electrons of
the lanthanides is expected to influence the anisotropy due to the strong spin -orbit coupling
and also t o cause structural distortio n [11, 12].
Over the past decade, several studies revealed the antimicrobial activity of the
cobalt ferrite and substituted cobalt ferrite nanoparticles (MxCo1-xFe2O4, M = Zn, Cu, Mn) ,
on pathogenic and multidr ug resistant bacterial strains [13-18].
An o verview of the literature shows only a few studies on the antimicrobial
properties of rare earth substituted cobalt ferrite nanoparticles CoFe 2-xRE xO4 (RE3+ = La3+,
Gd3+, Nd3+) [19, 20].

4 The proper choice of the synthesis method will plays a n important role on the
composition, structure and mor phology of the oxides , influencing also their biological
activities, including the antimicrobial one . Žalnėravičius et al. [21] has studied the
influenc e of the nanoparticle size on the antimicrobial activity and has reported that this
activity was increased with the decreasing of the particle size.
Rare earth substituted ferrite nanoparticles were synthesized by vario us methods
such as the coprecipitation me thod [21], the microemulsion method [9], the sol -gel
autoco mbustion techniques [12, 20], the forced hydrolysis in polyol [22] and the
sonochemical m ethod [11].
We hereby report a wet chemical method – so-called ‘’the wet ferritization method ”
– that takes place in a single step at low temperature [15, 23].
The ai m of this work was the synthesis of CoFe 1.8RE 0.2O4 (Re3+ = Tb3+, Er3+) ferrite
nanoparticles and t he investigat ion of the structural , morphological and magnetic
properties, and the antimicrobial activity against several bacterial and fungal strains, in
planktonic and biofilm growth state.

2 Experimental

2.1 Reagents

The iron(III) nitrate ( Fe(NO 3)3·9H 2O), the cobalt(II) nitrate ( Co(NO 3)2·6H 2O), the
terbium(III) nitrate ( Tb(NO 3)3∙6H 2O) and the erbium(III) nitrate ( Er(NO 3)3∙5H2O), all of
reagent quality (Merck) , were used in the synthesis procedure.

5
2.2 Synthesis of CoFe 1.8RE 0.2O4 (RE3+ = Tb3+, Er3+)

The metal nitrates (in the molar ratio 1.8Fe3+:1Co2+:0.2Re3+) were dissolved und er stirring
in distilled water . After the addition of the precipitating agent ( 25% NH 4OH) and raising
the pH to 10, a dark brown compound precipitate d. This suspension was maintai ned at 80
șC/7 h and the dark precipitate became magnetic. After filtering an d wash ing with distilled
water, the precipitate was dried on P 4O10.

2.3 Characterization Techniques

Powder X -ray diffraction investigations were performed by using a Rigaku Ultima IV
diffractometer in a parallel beam geometry, equipped with CuKα radiation (λ = 1.5406 Å),
Cross Beam O ptics (CBO) and graphite monochromator, and operated at 40 kV and 30
mA. The measurements were performed in 2θ range , between 10 ș and 80 ș with a scanning
rate of 5ș min-1 and a scan step increment of 0.02 ș. Phase identification was done by using
the HighScore Plus 3.0e software, connected to the ICDD PDF -4+2017 database. The
corresponding unit cell and space group were determined using McMaille method and full
profile Le Bail fit, assuming that all RE3+ ions occupy octahedral sublattice, 16d Wychoff
position. Lattice parameters were then refined by the Rietveld method. After removing the
instrumental contribution, the full -width at half -maximum (FWHM) of the diffraction
peaks can be interpreted in terms of crystalli te size and lattice strain. A Pseudo -Voigt
function was used to refine the shapes of the CoFe 1.8RE0.2O4 peaks and a Caglioti function

6 was used for FWHM approximation. Unit cell parameter , a, and oxygen occupancy, u, were
used to determine the cation radii [24]:

  2 125.0 3O T r u ar
(1)

 2 375.0 2 32
O O r u uar
(2)
where rT and rO are the average cation radius of the tetrahedral sublattice and octah edral
sublattice, respectively.
The inversion degree λ was estimat ed according to O’Neill method [25]:
t Fe t Co T r r r, ,3 2 1     
(3)

22.0 8.1, , ,3 3 2o Ln o Fe o Co
Or r r
r    
 
(4)
where λ, the degree of inversion , is the fraction of tetrahedral A sites occupied by Fe3+ and
s Xnr,
are the effective cation radi i as expressed by Shannon [26].
The microstructure of the CoFe 1.8RE 0.2O4 powder samples was studied by scanning
electron microscopy (FE -SEM), using a high resolution FEI QUANTA INSPECT F
microscope with field emission gun. The transmission electron micros copy (TEM/HR –
TEM) and selected area electron diffraction (SAED) analyses were performed by means a
TecnaiTM G2 F30 S -TWIN trans mission electron microscope , coupled with energy –
dispersive X -ray spectroscopy (EDX) component to investigate the elemental comp osition.
The a verage particle size for the CoFe 1.8RE 0.2O4 powder samples was determined using the
OriginPro 9.0 software by taking into account size measurements on 50 –60 particles from
TEM images of appropriate magnifications obtained from various microsc opic fields. The

7 TEM specimens were prepared by dispersion of the ferrite powders in ethanol by
ultrasonication and deposition of the resulting suspension onto a 400 mesh, holey carbon
coated film copper grid , and dried.
In order to confirm the formation o f the spinel ferrite phase, Fourier transform
infrared (FT IR) spectra were recorded on KBr pellets by a JASCO FTIR 4100
spectrophotometer in the frequency range 4000 –400 cm-1.
The magnetization versus field measurements were accomplished using a MPMS
SQUID magnetometer (Quantum Design) at two different temperatures ( 5 and 300 K) and
an applied magnetic field up to 5 Tesla.

2.4 Bioevaluation of the obtained nanoparticles

The antimicrobial activity of the obtained nanoparticles was evaluated on Gram -negativ e
(Escherichia coli ATCC 8739, Pseudomonas aeruginos a ATCC 27853), Gram -positive
(Staphylococcus aureus ATCC 6538, Enterococcus faecalis ATCC 29212) and fungal
(Candida albicans ATCC 10231) strains. The used quantitative assays allowed to establish
the minimal inhibitory concentration (MIC), indicating the efficiency of the obtained
nanoparticles against planktonic cells and minimal biofilm eradiation concentration
(MBEC), showing the anti -biofilm potential of the respective nanoparticles. In this purpose,
a stock suspension of 10 mg mL-1 in dimethyl sulfoxide (DMSO) was used to perform
serial binary dilutions, ranging from 1 mg to 0.002 mg mL-1, in 200 µL of Muller Hinton
broth distributed in 96 -well microplates. The respective plates were then inoculated w ith
microbial suspensions of 0.5 McFarland density (corresponding to 1.5 108 CFU mL-1).

8 Positive (microbial suspensions) and negative (sterile culture medium) controls were used.
After incubation at 37 oC for 24 h, the absorbance of the liquid cultures was read at 600 nm
(Apollo LB 911 ELISA reader). The MIC value was considered as the lowest concentration
of the tested nanoparticles that inhibited the planktonic growth, the absorbance value being
comparable to that of the negative control.
After the MIC re ading, the plates were emptied, and the biofilms adhered to the
plastic wells were fixed for 5 min with cold methanol, coloured for 15 min with violet
crystal and then resuspended with acetic acid 33%. The MBEC value was established by
measuring the optica l density of the coloured suspension at 490 nm and corresponded to the
lowest concentration of the tested nanoparticles that inhibited the development of microbial
biofilm [27].

3 Results and discussion

The CoFe 1.8RE 0.2O4 (RE3+ = Tb3+, Er3+) spinel ferrites were obtained through wet
ferritization – a chemical synthesis route belonging to the soft chemistry . The method
implies the decomposition of the polynuclear coordination compounds directly in the
reactio n medium [15].

3.1 Structural Characterization

The XRD diffraction patterns of CoFe 2-xRE xO4 (RE3+ = Tb3+, Er3+) powder samples
indicated the complete incorporation of the solutes into the CoFe 2O4 lattice with the

9 formation of the single phase cubic spinel structure, belonging to the space group Fd3m
(Fig. 1). This means that, for a substitution degree corresponding to x = 0.2, the solubility
limit of Tb3+, as well as that one of Er3+ on the octahedral B sites of the spinel network is
not yet reached. These results are not consistent with thos e reported previously by Kakade
et al. [28] and Prathapani et al. [12], who have found lower values of the solubility limit
corresponding to x < 0.15 and x < 0.03, respectively, in Er3+-doped cobalt ferrite powders
prepared by the sol -gel combustion method . On the other hand, our results are in agreement
with those reported by Cheng et al. [29] for their Er3+-doped cobalt ferrite films prepared
via sol-gel method . One can conclude that t he spread of the data regard ing the solubility of
RE3+ ions in the spinel structure of CoFe 2O4 is related to the preparation route and , for a
similar preparation method, it strongly depends on the synthesis/ thermal processing factors.
In o ur case, t he best fit was obta ined by using the ICDD card no. 01 -080-6487,
corresponding to CoFe 2O4. When Fe3+ ions onto the octahedral sites (C.N. = 6) are partially
replaced by Tb3+ / Er3+, an increase in the lattice parameter values is expected, taking into
account the values of the ionic radii of the substituting and substituted species, i.e. r(Tb3+) =
0.923 Å and r(Er3+) = 0.89 Å, relative to the Fe3+ ionic radius in high spin state, r(Fe3+) =
0.645 Å .26 Indeed , the lattice parameter a increases and, consequently, the unit cell sli ghtly
expands in the order CoFe 2O4 < CoFe 1.8Er0.2O4 < CoFe 1.8Tb0.2O4. On the other hand, due to
the higher atomic mass of Er and Tb solutes, relative to the atomic mass of the substituted
iron host -specie, the molecular mass of CoFe 1.8RE 0.2O4 becomes signi ficantly higher than
that one corresponding to the un -doped CoFe 2O4. In the case of the RE3+-doped
compositions, the increase rate of molecular mass due to the presence of heavy metals is

10 faster than that of the volume of the unit cell , so that the crystal lographic (theoretical)
density values decrease in the order CoFe 1.8Er0.2O4 > CoFe 1.8Tb0.2O4 > CoFe 2O4.
The increase in the mismatch of the ionic radii values of RE3+ solutes, and Fe3+ host
cations induces a slight decrease of the average crysta llite size and, concurrently, a slight
increase of the lattice microstrains. The values of the structural parameters are summarized
in Table 1. It is worthy to mention that our results are in good agreement with the data
reported by Kakade et al. [28] who also found an increase in the unit cell parameter and the
reduction in the crystallite size with the increase of the erbium content in their Er3+-
substituted nanocrystalline, cobalt -rich ferrite samples prepared by sol -gel autocombustion
method .
In a spinel structu re, the degree of inversion represents the fraction of tetrahedral
sites occupied by B ions. Therefore, in CoFe 2O4 the degree of inversion, , is related to the
fraction of Co2+ ions which occupy the tetrahedral sites of the spinel structure with the
gener al formula 
. The calculated value of the degree of
inversion  = 0.61, obtained for the pure CoFe 2O4 from ICDD card no. 01 -080-6487 , shows
that Co2+ cations are not found entirely onto the octahedral sites of the spinel lattice, as in a
typical inverse sp inel, but a certain fraction (1 - = 0.39) also occupies the tetrahedral sites.
As a result, the fraction of Fe3+ ions on the octahedral sites is higher than in a n ideal inverse
spinel with the theoretical formula (Fe3+)T[Co2+Fe3+]OO4.
It is well-known that RE3+ species exhibit a high tendency to occupy the octahedral
site because of their larger ionic radius than that of the ho st metal ions on the octahedral B
site (r(Co2+) = 0.745 Å and r(Fe3+) = 0.645 Å ) [30]. It seems very likely that t he entrance of

11 the larger Tb3+ and Er3+ cations into the octahedral sites of the spinel lattice induces internal
strains which are released by the partial migration of Co2+ ions from octahedral to
tetrahedral sites, concurrently with the opposite migration of an equal fraction of smaller
Fe3+ ions from the tetrahedral toward the octahedral sites. The higher ionic radius value of
the RE3+ solute, the lower will be the degree of inversion , i.e. the stronger will be the
preference of Co2+ ions for a tetrahedral c oordination. Consequently, for the composition
CoFe 1.8Er0.2O4, the inversion degree decreases to a  value of 0.25, while in the
CoFe 1.8Tb0.2O4 sample , the presence of the larger Tb3+ ions determines a clear evolution
toward a normal spinel structure ( = 0) (Table 1) . This tendency of cobalt ions reorde ring
from the octahedral sites (B) to the tetrahed ral sites (A) was also reported by Naik and
Salker [31] for a significantly lower Dy3+ amount (x = 0.03) incorporated in CoFe 2O4
lattice of their sample synthesized by sol -gel assisted autocomb ustion . However, in order to
better clarify the influence of RE3+ ions on the site occupancy of cations in the substituted
cobalt ferrite, these results should be sustained by experimental data provided by more
powerful experimental tools, as neutron diffraction. Two factors: particle size of ferrite and
size of dopan t RE3+ ion contribute to the level of internal microstrains in substituted cobalt
ferrite . Taking into account their opposite influence, the slightly higher value of internal
strains in CoFe 1.8Tb0.2O4 (Table 1) seems to indicate that the effect determined by a lower
particle size prevails over the effect of the larger Tb3+ ions in inducing a higher structural
relaxation by reducing the degree of inversion .

12
Table 1 Structural and microstructural parameters of CoFe 2O4 (from ICDD 01-080-6487) and synthesize d CoFe 1.8Er0.2O4 and
CoFe 1.8Tb0.2O4 powder samples
Structural / microstructural parameters Composition
CoFe 2O4 (ICDD 01-080-6487) CoFe 1.8Er0.2O4 CoFe 1.8Tb0.2O4
Unit cell
parameters
(cubic Fd3m) a = b = c (Å) 8.3554 8.3751 ± 0.0051 8.3775 ± 0.0018
(o) =  (o) =  (o) 90 90 90
No. molecules/unit cell, Z 8 8 8
Unit cell volume, V (Å3) 583.31 587.4 5 587.95
Oxygen occupancy, u 0.25664 0.258455 0.260472
Average cation radius of the tetrahedral
sublattice, rT (Å) 0.5251 0.5559 0.5857
Average cation rad ius of the tetrahedral
sublattice, rO(Å) 0.6549 0.64545 0.6305
Theoretical density, t (g/cm3) 5.343 5.809 5.766
Average crystallite size, < D> (nm) – 6.42 ± 0.63 5.77 ± 0.50
Microstrains (%) – 1.41 ± 0.54 1.57 ± 0.62
Degree of inversion 0.61 0.25 0
Formula obtained for the estimated
degree of inversion, 

Goodness of fit, 2 – 2.34845 1.49297
Average particle size, < dTEM> (nm) – 6.80 ± 1.81 5.91 ± 2.13 nm

13

Fig. 1 XRD patterns of spinel ferrites a CoFe 1.8Tb0.2O4 sample, b CoFe 1.8Er0.2O4 sample

3.2 FE-SEM and TEM/HRTEM Investigations

FE-SEM investigations revealed that both CoFe 1.8Tb0.2O4 and CoFe 1.8Er0.2O4 powders
consist of particles with sizes in the nanometric range, which exhibit a high agglomeration
tendency, so that even a rough est imation of the average particle size becomes a difficult
task (Figs. 2a and 2b).

14

Fig. 2 FE-SEM images of a CoFe 1.8Tb0.2O4 powder sample, b CoFe 1.8Er0.2O4 powder
sample

In order to overcome this drawback and to estimate more accurately the size and
details regarding the shape of the particles, TEM analyses were performed. T he TEM
images of Fig. 3 a and Fig. 4 a show that, both , CoFe 1.8Tb0.2O4 and CoFe 1.8Er0.2O4 consist of
primary polyhedral particles, with sizes bellow 10 nm. The determined average particl e size
(<dTEM>) is 5.91 nm for the CoFe 1.8Tb0.2O4 powder and 6.80 nm for the CoFe 1.8Er0.2O4
sample, as revealed the related particle size distribution histograms (insets of Fig. 3a and
Fig. 4a ). These values are close to those corresponding to the average crystallite size
presented in Table 1, which suggest the single -crystal nature of CoFe 1.8RE 0.2O4 particles
under investigation. The size of these particles, synthesized by the wet ferritization method ,
is significantly lowe r than the sizes reported for RE3+-substituted CoFe 2O4 powders
prepare d by other wet chemical routes, as the sol-gel a ssisted auto -combustion method [28,
32].

15 The HRTEM images of Figs. 3b and 4b , revealed long -range ordered fringes spaced
at 2.96 Å and 2.53 Å, corresponding to the crysta lline (2 2 0) and (3 1 1) planes of the
spinel lattice. The clearly defined diffraction rings, without any halo specific to a potential
amorphous phase, also suggest a high crystallinity, in spite of the nanometric size of both
particle types. The purity o f the powders was checked by qualitative EDX analyses. No
impurities were detected, so that the EDX spectra show only the host Co, Fe, Er, Tb and O
elements (Fig s. 3d and 4d). The presence of the C peak is determined by the carbon coating
of the Cu grid.

16 Fig. 3 a TEM image of CoFe 1.8Tb0.2O4 powder sample , b HRTEM image, c selected area
electron diffraction pattern , d EDX spectrum of CoFe 1.8Tb0.2O4 powder sample

Fig. 4 a TEM image of the CoFe 1.8Er0.2O4 powder sample , b HRTEM image , c selected
area elec tron diffraction pattern, d EDX spectrum of CoFe 1.8Er0.2O4 powder sample

3.3 FTIR Spectra

The formation of the spinel structure of CoFe 1.8RE 0.2O4 (RE3+ = Tb 3+, Er3+) is also
supported by FTIR analysis (Fig. 5). Two intense bands at about 570 –580 cm-1 (ν1) and

17 450–390 cm-1 (ν2) were due to the stretching vibrations of Fe -O and Co -O in tetrahedral
and octahedral sites, respectivel y [33]. The low intense bands between 1620 and 1370 cm-1
together with the broad band at about 3350 cm-1 have been assigned t o the H -O-H bonding
mode of adsorbed water molecules on the surface of the spinel oxides [34] .

Fig. 5 FTIR spectra of a CoFe 1.8Tb0.2O4 powder sample , b CoFe 1.8Er0.2O4 powder sample

3.4 Magnetic Measurements

Fig. 6 shows the magnetization curves ( M-H) for CoFe 1.8Tb0.2O4 and CoFe 1.8Er0.2O4
obtained at two different temperatures (5 K and 300 K) and an applied magnetic field of 5
T.

18

Fig. 6 The M -H curves at 5 K and 300 K for a CoFe 1.8Tb0.2O4 sample , b CoFe 1.8Er0.2O4
sample

Both samples show ferromagnetic behaviour with the saturation magnetization (M s)
which increases monotonous as the temperature decreases from 300 K to 5 K . The M s
estimated at 300 K (~ 60 emu/g for CoFe 1.8Tb0.2O4, and ~80 emu/g for CoFe 1.8Er0.2O4)
closely match with the previously reported values [9, 32]. The magneti c properties of rare
earth doped CoFe 2O4 ferrite strongly depend on the size and shape of the nanoparticles

19 which are closely related to the method of preparation [29] . The magnetic behaviour of the
samples consists in the formation of multiple magnetic domains generated by the cation
distribution on the tetrahedral (A) and octahedral (B) sites.
This characteristic is confirmed by M -H curves at low temperatures where the slope
changes are obvious. Normally, the cation distribution in substituted CoFe 2O4 ferrite is
(Fe3+)A[Co2+1-xMxFe3+]BO4. The super -exchange interactions (e.g., Fe3+(B)–O–Fe3+(B),
Fe3+(B)–O–Co2+(B), Fe3+(B)–O–Fe3+(A), Co2+(B)–O–Fe3+(A), and Fe3+(A) –O–Fe3+(A)) is
expected to be modified in RE3+ doped c obalt ferrite due to the appearance of RE3+–Fe3+
interactions (3d –4f coupling) [35, 36]. The co ntribution to the M s in CoFe 1.8Tb0.2O4 and
CoFe 1.8Er0.2O4 can be interpreted in terms of rearrangement of cations in tetrahedral and
octahedral sites, where some Co2+ ions can migrate to the tetrahedral sites by replacing an
equivalent amount of Fe3+ ions from tetrahedral to octahedral sites. A wide range of
experimental studies have shown that the magnetic properties of cobalt ferrite are affected
by pa rtial replacement of Fe ions with various RE ions [37 -40]. Another factor that could
induce changes in the magnetism of the two samples is the magnetic behaviour of RE3+
(both dopants have relatively high magnetic moments ~ 9.58µ B for Er3+ and ~ 9.72µ B for
Tb3+) and their distribution in the tetrahedral and octahedral sites [41] . The preferential
tenancy of rare earth ions towards the octahedral sites in CoFe 2O4 spinel lattice will result
in minimizing the number of Fe3+ ions at these sites and consequently the net magnetization
will decrease. For both compositions, the total magnetic moments were calculated. The
obtained values, 2.25 µ B for Tb3+ doped CoFe 2O4, respectively, 2.67 µ B for Er3+ doped
CoFe 2O4 are in agreement with those reported until now for th e rare earth doped cobalt
spinel ferrite, and very close to the theoretic magnetic moment of the CoFe 2O4 (3 µ B) [42,

20 43]. Considering that, the magnetic properties of the RE3+ doped CoFe 2O4 depend on the
unpaired 4 f electrons of RE ions [44], an increase in the CoFe 1.8Er0.2O4 magnetic moment
is well justified.

3.5 Antimicrobial Activity

The tested compounds exhibited a higher efficiency against P. aeruginosa
(MIC=MBEC=0.125 µg mL-1) and E. faecalis in planktonic growth (MIC=0.5 µg mL-1), as
compared to t he other tested strains, for which higher MIC and MBEC values, of 1 µg mL-1
were recorded (Table 2).

Table 2 MIC and MBEC values of the tested nanoparticles against bacterial and fungal
strains
MIC S. aureus E. faecalis E. coli P. aeruginosa C. albicans
CoFe 1.8Tb0.2O4 1 0.5 1 0.125 1
CoFe 1.8Er0.2O4 1 0.5 1 0.125 1
MBEC S. aureus E. faecalis E. coli P. aeruginosa C. albicans
CoFe 1.8Tb0.2O4 1 1 1 0.125 1
CoFe 1.8Er0.2O4 1 1 1 0.125 1

The tested nanoparticles exhibited the same spectrum and intensit y of the
antimicrobial activity. Taking into account that the microbial biofilms are normally much
more resistant to antimicrobial agents than their planktonic counterparts, requiring higher
concentrations of active drugs, a very promising result is the fa ct that, exempting E.

21 faecalis , the tested nanoparticles exhibited the same efficiency against planktonic and
adhered bacteria [45] .
The obtained results demonstrate the promising potential of the obtained
nanoparticles for the development of antimicrobial applications, such as for the
development of novel anti -biofilm agents or for the incorporation of the respective
nanoparticles in different materials, in order to increase their resistance to microbial
colonization and biofilm development, and therefore, to avoid the negative consequences of
biofilm formation, in the industrial, ecological and medical fields.

4 Conclusions

The synthesis of Tb3+ – and Er3+ – substituted cobalt ferrites through wet ferritization
method using temperatures below 100 oC lead s to the formation of spinel oxide single
phase.
The lattice parameter a of the samples was found 8.3751 Å for CoFe 1.8Er0.2O4 and
8.3775 Å for CoFe 1.8T0.2O4. The inversion degree  is 0.25 for CoFe 1.8Er0.2O4 while for
CoFe 1.8Tb0.2O4, the presence of the larger Tb3+ ions leads to a normal spinel structure ( =
0). The average particle size value (6.80 nm for CoFe 1.8Er0.2O4 and 5.91 nm for
CoFe 1.8Tb0.2O4) estimated from the particle size distribution is close to the value of the
average crystallite size ( 6.42 nm for CoFe 1.8Er0.2O4 and 5.77 nm for CoFe 1.8Tb0.2O4)
calculated from the XRD data, which proves the single crystal nature of the particles.
FTIR spectra sustained the formation of spinel ferrites.

22 Both CoFe 1.8RE 0.2O4 samp les are ferromagnetic; the saturation magnetization (Ms)
increases with temperature decreasing from 300 K to 5 K. The Ms value at room
temperature is ~ 60 emu/g for CoFe 1.8Tb0.2O4, and ~80 emu/g for CoFe 1.8Er0.2O4.
The antimicrobial activity assays revealed a similar efficiency of the two types of
CoFe 1.8RE 0.2O4 ferrite nanoparticles against the tested Gram – positive and Gram –
negative bacteria and fungal strains, both in planktonic and biofilm state. These re sults
highlight the promising potential of these types of nanoparticles for the development of
novel anti -biofilm agents .
Hence, this work shows the possibility of easy getting a wide variety of rare earth
substituted spinel ferrites with a good control o f phase purity and stoichiometry by a
''chimie douce" process – the wet ferritization method.

Acknowledgments The authors from “Ilie Murgulescu” Institute of Physical Chemistry
acknowledge the support of the “Materials Science and Advanced Characterizatio n
Methods” Programme , financed by the Romanian Academy . I. Mindru and D. Gingasu
would like to thank Dr. Luminita Patron for their support and expertise.

Compliance with Ethical Standards

Conflict of interest The authors declare that they have no confl ict of interest.

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